1. Field of the Invention
The present invention relates to elastic wave filters and communication devices equipped with such elastic wave filters. In particular, the present invention relates to an elastic wave filter that applies elastic waves, such as a surface acoustic wave filter and an elastic boundary wave filter, and also relates to a communication device equipped with such an elastic wave filter.
2. Description of the Related Art
As an example of a bandpass filter having passband frequency ranging from several tens of MHz to several GHz, a surface acoustic wave filter is known. Due to having a compact, lightweight structure, surface acoustic wave filters have been used in portable communication devices in recent years.
Although there are various types of surface acoustic wave filters, the type commonly used in a front end of a portable communication device is a longitudinally coupled resonator type surface acoustic wave filter having two reflectors arranged on a piezoelectric substrate in a transmitting direction of a surface acoustic wave and also having input IDTs (interdigital transducers) and output IDTs arranged alternately between the two reflectors. The longitudinally coupled resonator type surface acoustic wave filter is characterized in that the insertion loss in the frequency within a band is small and that the conversion between balanced signals and unbalanced signals can be achieved readily.
The principle of operation in the longitudinally coupled resonator type surface acoustic wave filter is as follows.
An input electric signal is converted to a surface acoustic wave by the input IDTs so that a standing wave of the surface acoustic wave is generated between the two reflectors. The energy of the generated standing wave is converted to an electric signal by the output IDTs so that an output signal is generated. In this case, the conversion efficiency between the electric signal and the surface acoustic wave in the input IDTs and the output IDTs has a frequency characteristic, and moreover, the surface acoustic wave reflection efficiency of the reflectors also has a frequency characteristic. Thus, the longitudinally coupled resonator type surface acoustic wave filter has a bandpass characteristic that only transmits signals that are within a certain frequency range.
In order to increase the amount of signal attenuation outside the passband, a surface acoustic wave filter is used which has two or more longitudinally coupled resonator type surface acoustic wave filter elements that are cascade connected to each other on a piezoelectric substrate. By cascade connecting multiple longitudinally coupled resonator type surface acoustic wave filter elements, the signals outside the passband are sequentially attenuated by the corresponding longitudinally coupled resonator type surface acoustic wave filter element, whereby the amount of signal attenuation outside the passband is increased (see, for example, Japanese Unexamined Patent Application Publication No. 2002-9587).
FIG. 1 illustrates a surface acoustic wave filter 150 having two longitudinally coupled resonator type surface acoustic wave filter elements 106, 112 disposed on a piezoelectric substrate 100 and cascade connected to each other.
The longitudinally coupled resonator type surface acoustic wave filter element 106 includes two reflectors 101, 105 between which three IDTs 102, 103, 104 are aligned in a transmitting direction of a surface acoustic wave. Similarly, the longitudinally coupled resonator type surface acoustic wave filter element 112 includes two reflectors 107, 111 between which three IDTs 108, 109, 110 are aligned in a transmitting direction of a surface acoustic wave. The reflectors 101, 105, 107, 111 are periodical gratings. The IDTs 102 to 104, 108 to 110 are interdigitating comb shaped electrodes.
The IDT 102 and the IDT 108 are connected to each other via a wire 113, and the IDT 104 and the IDT 110 are connected to each other via a wire 114, whereby the longitudinally coupled resonator type surface acoustic wave filter elements 106, 112 are cascade connected to each other.
Wires 115 to 122 are provided as conductors between pads 123 to 130 (128 not included) and the IDTs 102 to 104, 108 to 110. The pads 123 to 127 function as grounding pads that are grounded. On the other hand, the pad 129 functions as an input pad to which an input voltage is applied. The pad 130 functions as an output pad in which an output voltage is generated.
The reflectors 101, 105, 107, 111; the IDTs 102 to 104, 108 to 110; the wires 113 to 122; and the pads 123 to 130 (128 not included) together define a metallic film pattern provided on the piezoelectric substrate 100. For example, the metallic film pattern is produced by a thin film micromachining process, which may be a vacuum film forming process, a photolithography process, an etching process, or a lift off process.
FIG. 2 illustrates a surface acoustic wave filter 250 which is provided with a function for conversion between an unbalanced signal and a balanced signal and has two longitudinally coupled resonator type surface acoustic wave filter elements 206, 212 disposed on a piezoelectric substrate 200 and cascade connected to each other. Since the surface acoustic wave filter 250 and the surface acoustic wave filter 150 share many common features, the differences from the surface acoustic wave filter 150 will be mainly described below. Therefore, description of the reflectors 201, 205, 207, 211; IDT 203; and wires 215-221 will be omitted below.
An IDT 209 is divided into two IDT segments. The IDT 209 generates a balanced signal. Pads 223 to 227 are grounded. When an unbalanced input signal is input to a pad 229, balanced output signals are generated in a pad 230 and a pad 231.
In the surface acoustic wave filter 250, the polarities of an IDT 202 and an IDT 204 are inverted, and the polarities of an IDT 208 and an IDT 210 are also inverted. Thus, a connection wire 213 and a connection wire 214 transmit reversed phase signals. This technique is effective for improving the degree of balance of the balanced output signals of the surface acoustic wave filter 250. Alternatively, the function for conversion between an unbalanced signal and a balanced signal in the surface acoustic wave filter 250 can be sufficiently achieved without using this technique. In that case, the IDT 202 and the IDT 204 are given the same polarity and the IDT 208 and the IDT 210 are also given the same polarity so that the connection wire 213 and the connection wire 214 transmit in-phase signals. However, using the above mentioned technique is advantageous in that the degree of balance of the balanced output signals is improved.
Examples of surface acoustic wave filters have just been described above. As a similar filter, an elastic boundary wave filter is known. Similar to surface acoustic wave filters, an elastic boundary wave filter includes reflectors and IDTs made of a metallic film disposed on a piezoelectric substrate. For example, the elastic boundary wave filter has filter electrodes including IDTs and reflectors made of, for example, Al on a surface of a piezoelectric single crystal plate, and a film having a sufficient thickness and made of, for example, SiO2 on the filter electrodes. The film has an elastic constant or density that is different from that of the piezoelectric single crystal. Although the operation and the structure are substantially the same as those of surface acoustic wave filters, the elastic boundary wave filter additionally has a solid layer disposed over the surface of the piezoelectric substrate. The elastic boundary wave filter operates based on interaction between the IDTs and an elastic wave (elastic boundary wave) transmitted through the boundary between the piezoelectric substrate and the solid layer. In contrast to the surface acoustic wave filter which requires a package having a cavity to prevent the surface of the substrate from being restrained, the elastic boundary wave filter is advantageous in that it does not require such a package having a cavity since the wave is transmitted through the boundary plane between the piezoelectric single crystal substrate and the film.
In short, a surface acoustic wave filter operates based on a transmission of a surface acoustic wave through a surface of a piezoelectric substrate, whereas an elastic boundary wave filter operates based on a transmission of an elastic boundary wave through a boundary between a piezoelectric substrate and a solid layer. The principle of operation of the two is basically the same, and the design approach of the two is similar.
In the specification of the present invention, the term “elastic wave filter” will be used as a generic term to refer to a filter, such as the surface acoustic wave filter and the elastic boundary wave filter, which applies an elastic wave (e.g. Rayleigh wave, SH wave, pseudo surface acoustic wave, Love wave, Sezawa wave, Stonely wave, boundary wave). Furthermore, the term “longitudinally coupled resonator type elastic wave filter” will be used as a generic term to refer to a longitudinally coupled resonator type surface acoustic wave filter and a longitudinally coupled resonator type elastic boundary wave filter.
High frequency bandpass filters like the elastic wave filters require good impedance matching. A filter having bad impedance matching in the input-output terminals, that is, a filter having large signal reflection in the input-output terminals, is subject to bad (large) insertion loss since the signals are lost due to reflection. Moreover, the signals reflected in the input-output terminals of the filter re-enter other electronic components connected to the filter, which could lead to failures, such as a transmission error of a circuit.
The terms “good impedance matching”, “small signal reflection”, and “small VSWR (voltage standing wave ratio)” are all used synonymously with one another. If the impedance matching is good, the signal reflection is smaller and the VSWR is also smaller. A small VSWR implies that the signal reflection is small, which means that the impedance matching is good.
In an elastic wave filter having a plurality of cascade connected longitudinally coupled resonator type elastic wave filter elements disposed on a piezoelectric substrate, the impedance matching in the cascade connected portion between the longitudinally coupled resonator type elastic wave filter elements affects the impedance matching of the entire elastic wave filter. Specifically, if the impedance matching in the cascade connected portion between the longitudinally coupled resonator type elastic wave filter elements is bad, and if signal reflection occurs in this cascade connected portion, the reflected signal is released outward from the filter as a reflection wave of the elastic wave filter.
Concerning the cascade connected portion between the longitudinally coupled resonator type elastic wave filter elements, the impedance of one of the longitudinally coupled resonator type elastic wave filter elements with respect to the cascade connected portion and the impedance of the other longitudinally coupled resonator type elastic wave filter element with respect to the cascade connected portion ideally have a complex conjugate relationship. If the two have a complex conjugate relationship, the impedance matching in the cascade connected portion is complete, meaning that signal reflection in this portion will not occur at all.
However, under present circumstances, the impedances of the longitudinally coupled resonator type elastic wave filter elements with respect to the cascade connected portion tend to become capacitive (i.e. the imaginary impedances become negative) due to the parasitic capacitance between cascade connected wires and a grounding pattern, meaning that an ideal complex conjugate state (in which one of the imaginary impedances is positive and the other imaginary impedance is negative) is difficult to attain. This is the factor that increases the signal reflection in the cascade connected portion between the longitudinally coupled resonator type elastic wave filter elements. As a result, the VSWR characteristic of an elastic wave filter having cascaded connected longitudinally coupled resonator type elastic wave filter elements is deteriorated. This problem is more noticeable in a case where the piezoelectric substrate has a large relative dielectric constant since the parasitic capacitance between the cascade connected wires and the grounding pattern increases in proportion to the relative dielectric constant. Moreover, this problem becomes more noticeable as the frequency within the passband of the filter becomes higher since the current flowing into the parasitic capacitance increases in proportion to the frequency within the passband.